2. Field of the Invention
The present invention relates to a compound semiconductor device, and more particularly to a heterojunction bipolar transistor (hereinafter referred to as HBT).
2. Description of the Art
As a result of the progress of crystal growth techniques, such as a molecular beam epitaxy (hereinafter referred to as MBE) method and a metal organic chemical vapor deposition (hereinafter referred to as MOCVD) method, the development of semiconductor devices utilizing heterojunctions is being pursued. A heterojunction device is formed by joining semiconductor materials which are different from each other.
For example, a HBT is a semiconductor device which utilizes a heterojunction between an emitter and a base of a bipolar transistor. The aforesaid HBT has several advantages as compared with a conventional homojunction bipolar transistor, which is formed by using a single semiconductor material.
First, the semiconductor material constituting the emitter layer has an energy gap wider than that of the semiconductor material constituting the base layer. Therefore, the impurity concentrations of the emitter and base regions may be set independently without lowering the efficiency of the emitter injection.
Further, the base resistance may be decreased. The base layer may be formed less thick since the impurity concentration of the base layer can be increased. In addition, since the impurity concentration of the collector layer can also be reduced, the collector capacitance thereof may be reduced accordingly.
Because of the above advantages, the HBT achieves superior performance in the high frequency and switching characteristics compared with the conventional homojunction bipolar transistor.
FIG. 1 is a sectional view of the conventional HBT having a semi-insulating GaAs substrate 1, an N.sup.+ type GaAs collector contact layer 2, an N type GaAs collector layer 3, a P.sup.+ type GaAs base layer 4, an N type AlGaAs emitter layer 7, an N.sup.+ type GaAs emitter contact layer 9, an emitter electrode 10, a base electrode 11, and a collector electrode 12.
A HBT, constructed as above, has a limitation in that the base resistance cannot be reduced beyond a certain level, even though the impurity concentration of the base layer is high, because the base layer is thin.
To decrease the base resistance, a built-in field may be formed by varying the material content of the base layer. For example, Al.sub.x Ga.sub.1-x A.sub.s may be used as the material of the base layer, where the value of x decreases when moving from the emitter layer to the base layer. The carrier mobility in the base region is increased by the built-in field and thus, the time for the carrier to pass the base layer is decreased, so that the base layer can be formed more thick. However, this method still has a limitation in reducing the base resistance.
Furthermore, in the above HBT, the collector capacitance is reduced by etching a portion of the base and collector layers and thus decreasing the junction area between the base and the collector layers. But, a large capacitance still exists, thereby impeding high speed operation at high frequencies.
FIG. 2 illustrates another kind of HBT which has graded layers 6 and 8 grown according to an MBE method so that the concentration of aluminum component changes gradually to remove a steep potential barrier occurring due to the heterojunction in the HBT shown in FIG. 1. The HBT shown in FIG. 2 has an enhanced current gain compared with that shown in FIG. 1 due to a gentle potential barrier in the heterojunction.
However, this HBT also has the same disadvantage as that of the HBT shown in FIG. 1 including a high base resistance and a large collector capacitance.
The same reference numerals are used in FIGS. ; and 2 to designate the same parts. Grade layer 6 is interposed between P.sup.+ type GaAs base layer 4 and N type AlGaAs emitter layer 7, and the aluminum concentration thereof varies gradually so that layer 6 has an N-type GaAs formation near base layer 4 and N.sup.+ type AlGaAs formation near emitted layer 7. Graded layer 8 is interposed between N type AlGaAs layer 7 and N.sup.+ type GaAs contact layer 9 and the aluminum concentration thereof varied gradually so that layer 8 has an N type AlGaAs formation near layer 7 and an N.sup.+ type GaAs formation near layer 9.
Undoped GaAs spacer 5 is interposed between graded layer 6 and base layer 4, and prevents an impurity contained in the base layer from diffusing into graded layer 6 when the graded layer is formed. (In the case of a P.sup.+ type GaAs base layer, the impurity includes Mg.sup.+ or Be.sup.+.